For monitoring the operational readiness of optical networks, especially for the transmission of optical wavelength multiplexed signals, it is known to use an optical monitoring channel. For example, a unique wavelength can be used for this purpose in a DWDM signal. In this way, the information necessary for the complete management of the network can be transmitted between the different network nodes. Among other things, this monitoring channel can also be used to determine whether the optical transmission line between two network nodes is intact or broken. For this purpose, a network node is tested to determine whether an optical signal of the monitoring channel has been received or not. For example, here a signal can be transmitted to the relevant adjacent network node on the monitoring channel, optoelectrically converted and received in this node, and as a response, a corresponding signal can be transmitted back to the querying node on the monitoring channel. If the querying network node does not receive a reply signal within a predetermined time, then it concludes that there is a break in the transmission line.
Thus, on the subscriber side, the monitoring of a transmission line between a network node and an end subscriber requires an active device that can transmit a reply signal to a network node as a response to a querying signal.
Here, however, the expense for an active device, which is normally provided at the end point of the transmission line by the provider, who is responsible for the transmission line, is on the subscriber side and optionally must be paid for by the end subscriber.
From DE 100 19 814 A1, a method for monitoring the operation of optical supply fibers is known, in which an optical signal, in addition to the usable signal, i.e., an additional wavelength (or an additional wavelength range), is transmitted outside the spectrum of the usable signal from a central optical device to a decentralized optical device, separated from the usable signal in the decentralized device by means of a passive optical coupler, and transmitted back in the direction towards the central device. The central optical device detects the additional signal and recognizes a break in the transmission line if this signal is no longer present.
This method enables the monitoring of the line from a remote node, with only passive optical couplers being required in the decentralized device. However, a disadvantage is the expense for an additional optical transmitter element, as well as for the wavelength-selective passive coupler, as well as the fact that a certain wavelength range must be reserved for the additional monitoring signal, and therefore this range is not available for the transmission of usable signals.